Poly(ethylene 2,6-naphthalenedicarboxylate), referred to as PEN, is widely used as an extrusion and injection molding resin because of its good heat resistance, high glass transition temperature and gas barrier properties. PEN may be used in the fabrication of various articles for household or industrial uses including appliance parts, containers, automobile parts, films, and fibers.
PEN polymers prepared in conventional melt phase condensation polymerization reactions are generally pelletized by cutting strands of polymer under water. Such pellets are amorphous and are generally crystallized before processing. Further processing includes solid state polycondensation to increase the molecular weight of the polymer, film extrusion, fiber spinning, and injection molding into desired shapes or objects. Sticking or agglomeration may occur during these processing operations if the pellets are not crystallized.
Although most polyesters such as poly(ethylene terephthalate) are easy to dry at elevated temperatures prior to crystallization in batch dryers or other conventional continuous processing equipment, PEN pellets containing moisture or other volatile components tend to explode in a popcorn-like behavior when heated to elevated temperatures resulting in deformed and non-uniform pellet shapes which create materials handling problems.
One proposed solution to this problem for PEN is described in U.S. Pat. No. 4,963,644. This patent teaches that amorphous PEN pellets may be treated at 80.degree. to 140.degree. C. in the presence of a stream of an inert gas or under a vacuum to devolatilize the PEN pellets over a period of about 6 hours. Subsequently, the pellets are heated at 150.degree.-260.degree. C. to crystallize them. Although this process is effective, it is time consuming and therefore inefficient and expensive.
U.S. Pat. No. 4,798,883 does not address the drying problems associated with PEN but claims that PEN polymers containing small amounts of polyethylene glycol compounds crystallize more rapidly than unmodified PEN.
U.S. Pat. No. 4,374,974 describes the solid state polymerization of certain poly(ethylene terephthalate) homo- and copolyesters but does not disclose polyesters containing high levels of naphthalenedicarboxylic acid; nor does U.S. Pat. No. 4,374,974 address the problems associated with the drying of such polymers.
U.S. Pat. No. 3,960,817 describes a process for the solid phase polycondensation of thermoplastic polyesters. Although 2,6-naphthalenedicarboxylic acid is disclosed as a possible reactant, PEN and its attendant drying problems are not mentioned.